1. Field of the Invention
The present invention relates to a DC motor and, more particularly, to an inner rotor type DC motor with a cup-shaped stator. The present invention also relates to a DC fan formed from the DC motor.
2. Description of the Related Art
FIG. 1 shows a conventional DC fan including a housing 7, a DC motor 8, and an impeller 9. The housing 7 has a compartment 71 for receiving the DC motor 8. The DC motor 8 includes a stator 81 fixed to an inner periphery of the housing 7. The DC motor 8 further includes a rotor 82 rotatably received in the compartment 71. The stator 81 of the DC motor 8 includes an annular permanent magnet 811 surrounding the rotor 82. The stator 81 further includes a brush 812 adjacent an end of the permanent magnet 811 and electrically connected to a DC power source. The rotor 82 of the DC motor 8 includes a shaft 821, a winding core 822, and a converter 823. An end of the shaft 821 extends beyond the housing 7 and is coupled to the impeller 9. The winding core 822 is mounted around the shaft 821 and includes an outer periphery facing the permanent magnet 811 with an air gap formed between the winding core 822 and the permanent magnet 811. Electricity passing through the coil of the winding core 822 interacts with a magnetic field created by the permanent magnet 811. The converter 823 is also mounted around the shaft 821 and electrically connected to the coil of the winding core 822. The converter 823 has an outer periphery for sliding, electrical contact with the brush 812.
In operation, when DC power is supplied from the DC power source to the brush 812 of the stator 81, the DC power is transmitted through the converter 823 of the rotor 82 to the coil of the winding core 822. Electric current generated in the coil by the DC power interacts with the magnetic field created by the permanent magnet 811, driving the rotor 82 to rotate relative to the stator 81. The speed of the rotor 82 can be decided by controlling the DC power to modulate the current in the coil of the winding core 822.
However, the converter 823 is parallel to the shaft 821 and includes a plurality of spaced converter plates, such that sparks are liable to occur between the brush 812 and the converter 823 when the brush 812 moves from one of the converter plates to an adjacent converter plate. At the same time, a noise signal adversely affecting system self-control is apt to be generated. Further, the brush 812 must be in tight contact with the outer periphery of the converter 823 to assure electrical connection therebetween. To avoid adverse affect to the electrical connection between the brush 812 and the converter 823 by accumulated carbon resulting from sparks, the worn-out brush 812 must be periodically replaced, and the outer periphery of the converter 823 must be periodically cleaned. Further, although the speed of the rotor 82 can be decided by controlling the DC power, noise signals are apt to be generated during DC power transmission between the brush 812 and the converter 823, for the brush 812 moves between the converter plates. As a result, the DC fan can not be utilized in products requiring precise control of the speed of the fan.
FIG. 2 shows another conventional DC fan. The DC motor 8 of the DC fan of FIG. 1 is replaced by a DC motor 8′ having a cup-shaped rotor 82′ in the DC fan of FIG. 2. The cup-shaped rotor 82′ includes a cup-shaped coil 822′ and a shaft 821′ extending through the cup-shaped coil 822′ along an axis of the cup-shaped coil 822′. The cup-shaped coil 822′ is formed by winding at least one conductive wire. An annular permanent magnet 811′ of a stator 81′ of the DC motor 8′ is mounted in the cup-shaped coil 822′ with a shaft 821′ extending through the permanent magnet 811′. An inner periphery of the cup-shaped coil 822′ faces the permanent magnet 811′ with an air gap formed between the cup-shape coil 822′ and the permanent magnet 811′. The cup-shaped rotor 82′ possesses characteristics including low rotating inertia and high energy conversion rate such that the sensitivity in speed control of the DC fan can be enhanced. However, the light-weight structure reduces the rotational stability, while the DC motor 8′ still requires a brush 812′ and a converter 823′ to proceed with transmission of DC power. Namely, the disadvantages of the brush 812 and the converter 823 of the DC fan of FIG. 1 still exist.
Further, when the load of the above-mentioned DC motors 8, 8′ is changed, an additional sensing device such as a mechanical governor, an electronic governor, a speed signal generator, or an optical encoder is required to feed back the actual speed of the DC motors 8, 8′, so that the voltage of the DC power can be adjusted to drive the DC motors 8, 8′ to rotate at an expected speed. However, the manufacturing costs of the DC motors 8, 8′ and the DC fan formed from the DC motors 8, 8′ are increased, while the accuracy of the sensing device affects the controlling quality of the speed.
Thus, a need exists for a DC motor allowing precise speed control while allowing easy assembly and manufacture at low costs.